There is a vehicular control system using an angular velocity sensor. The vehicular control system includes a vehicular stability control system that maintains a vehicle in a normal state by detecting sideways skids and thereby properly controlling brakes or torques of wheels. The vehicular control system further includes four-wheel steering-angle control system that controls steering angles of front or rear wheels. These vehicular control systems detect vehicular abnormal states such as skids using signals of the angular velocity sensor, so enhancement in reliability of the signals of the angular velocity sensor is required.
Many of the sensors for vehicular control are provided redundantly as hardware components for enhancing their reliability. Namely, the hardware structure includes two or more same sets of the sensors. Here, two outputs of the two sets of the sensors are monitored in differences with each other to thereby detect their failure.
Further, the angular velocity sensors include individual deviations from others with respect to their detection sensitivity or zero-point offset, so threshold values for detecting the failure have to be set to include margins that absorb the deviations. Sensitivity errors or zero-point offset errors can be detected without problems while the outputs of the angular velocity sensors significantly exceed the threshold values. In contrast, minor failures that slightly vary the angular velocity signal level cannot be detected.
To deal with this issue, Patent Document 1 describes addition of a differentiation circuit, which differentiates outputs of the angular velocity sensor on a time basis, to thereby additionally monitor the differentiated signals for enhancing accuracy of failure diagnosis.                Patent Document 1: JP-2002-537549A (WO00/48883, U.S. Pat. No. 6,625,527 B1)        
However, this failure monitoring using the differentiation circuit requires addition of the circuit and also needs to process the differentiated signals in addition to the usual angular velocity signals, so this monitoring poses a disadvantage of increase in loads to process signal.
For instance, when a yaw rate is slightly generated during traveling on an expressway, the level of the angular velocity signal significantly decreases. It thereby becomes difficult to detect the sensitivity errors even using the differentiated signals. Further, since the differentiation circuit increases gains as the frequency increases, differentiated angular velocity signals are apt to receive influence of resonance due to a sensor's mechanical structure. This can be also a cause for making it difficult to detect the sensitivity errors.